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Thread: prism by decentration on aspherics?

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    OptiBoard Professional yzf-r1's Avatar
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    prism by decentration on aspherics?

    I have just received my new glasses from the lab. I have prism incorporated in my Rx (1 base up OD and 1 base down OS).

    The lenses i have ordered are Nikon NL4 AS (n=1.67). The lab have decentred the lenses to achieve the prism. Is this a correct method of doing it or should they have worked prism, considering the lens is an aspheric?

    Many thanks.

    Yahya

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    Optimentor Diane's Avatar
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    Yahya,

    What is your prescription?

    Diane
    Anything worth doing is worth doing well.

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    OptiBoard Professional Mike Fretto's Avatar
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    My understanding is that perscribed prism can be ground in, but you cant simply decenter to achieve the perscribed amount of prism. The patient may be looking through the right amount of prism but they would be looking through the aspheric portion of the lense designed for peripheal vision.
    Mike

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    Master OptiBoarder Darryl Meister's Avatar
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    Mike is exactly correct. You cannot decenter to induce prescribed prism in an aspheric, since you would be moving the "center" of the asphericity away from the line of sight. Similarly, you cannot grind prism for decentration.

    Best regards,
    Darryl

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    OptiBoard Professional yzf-r1's Avatar
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    my RX

    Diane,

    Diane said:
    Yahya,

    What is your prescription?

    Diane
    My prescription is
    OD -6.25/+1.25x120 1^ UP
    OS -6.25/+1.00x57.5 1^DOWN

    Thanks

    Yahya

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    OptiBoard Professional yzf-r1's Avatar
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    Unhappy my new spex (reworked) are here

    I picked up my new spectacles today, after the factory reglazed them. I think they have decentred the lenses again to achieve the vertical prism, but find this rather odd since they were sent back for that reason. Maybe i am not checking the prism correctly?

    Can someone show me what i should be doing in order to verify whether the prism is worked or lenses decentred???

    Any comments highly appreciated.

    Yahya.

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    Bad address email on file John R's Avatar
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    I'll try...
    What you need to do is dot the lenses where your PD is on datum. You should then be able to read the prism on a focimeter. But what you may need to bear in mind is that to get the prism by dec will only require 2m/m in the right & less than 2 m/m in the left. So you can see if they are glazed only slighty wrong you will get unwanted prism which could well be cancelling out your required prism.

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    OptiBoard Professional yzf-r1's Avatar
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    Question Checking prism

    John,

    I understand the method you have outlined. However, it will succeed in determining the amount of prism regardless of whether it was achieved using decentration or whether prism was surfaced.

    My main problem is that I am now wearing aspheric lenses which have been decentred (allegedly) to achieve prism. Incidentally, i am happy with the size of the prism.

    I would like some comment from fellow optiboarders on the method i use to determine whether prism has been worked.

    I look at the image of a line through the lens, and if i can get the image to line up with the actual line as seen outside the the lens, then the prism is by decentration. If the image of the line is shifted to one side, and i cannot ine them up by moving the lens from side to side, then prism has been worked. (i'm sure this isn't the clearest explanation, but i am having problems with my webspace, so i havent been able to upload an illustration of this)

    All comments highly appreciated.

    Yahya

    PS: I was also hoping to upload another document which gives details of what the factory have done (makes very little sense to me, but those in labs will certainly understand it). If this is of any help, i can email it to whoever requests it.

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    Master OptiBoarder Jeff Trail's Avatar
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    One way to determine if it was decentered (the apex) is to check the power on center and off center... I'll try to explain it..when you have an aspherical design the only place where you get a true power is at the apex of the curves as now as you cross the merridians of curve you will get a fluxuation of power (oblique, as well as axes change) so technically if the prism was ground at a cooridant from the apex than if you slid that prism oc onto the dead center in your lensometer than you would have a variation of power and axis from that which was requested.. the greater the power of amount of prism the higher the change...one thing to remember is you are deviating the ray but not the optical center of the apex of the aspherical surface (front)

    Does that make sense? .. well technically you have prism say 2 BU/2BD ... now the lens, if aspherical should be positioned when be marked up and blocked on center.. and than the back surface is ground with the oc on that side "pushed" up/down to get that degree of prism... now if I placed those lens into my lensometer and physically moved the surface to neutralize that prism than I should NOT be able to get the exact power and axis I wanted.. that would be down at the apex of the front surface..

    How about that version.easier to understand?

    Than again maybe someone else can sift through my gobble gook and maybe translate it into easier terms:p


    Jeff

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    Bad address email on file John R's Avatar
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    Re: Checking prism

    yzf-r1 said:
    I was also hoping to upload another document which gives details of what the factory have done (makes very little sense to me, but those in labs will certainly understand it). If this is of any help, i can email it to whoever requests it.
    Ok here goes thanks to Yahya who has mailed me the copies i can now post what he got.



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    Bad address email on file John R's Avatar
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    As we can see in the Highlighted areas Prism is ordered but has not been surfaced.
    I wonder if the laying off program has decided not to work the prism as it can easily be obtained by decentration, even though they are aspheric lenses. Maybe someone with more knowledge of laying off programs could shed some light on whether this is possible.

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    OptiBoard Professional yzf-r1's Avatar
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    Re: Re: Checking prism

    John R said:
    Ok here goes thanks to Yahya who has mailed me the copies i can now post what he got.
    Thanks alot John for posting the image up. I am still plugging away at finding out why my own uploader wont work.

    Yahya

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    Objection! OptiBoard Gold Supporter shanbaum's Avatar
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    Exclamation Can you feel it?

    I'm not an expert at reading DVI worktickets, but it certainly looks to me as though the only way you're going to obtain the prescribed prism on this job is if it's spotted in, which is to say, obtained by decentration.

    On the other hand, consider the following:

    1. The amount of decentration required to produce your one diopter in each eye is less than 2mm. I don't recognize the lens "Nikon IV" - could it be the same as what we call the "NL4" over here? In any case, we don't have anything like a 0.50 base (in 1.66), so I can't say for sure, but I would guess that the spherical zone on that lens could be greater than the "B" measurement of your frame (28mm??? freakin' Europeans!) in which case the 2mm vertical offset would have no impact on the optical performance of the lens in the vertical meridian, and I would expect at most an indescernable impact on lateral performance.

    2. One diopter of prism on your Rx is flirting with the ISO standard for PRP placement, which for your Rx is 0.37D plus the amount induced by 1mm (0.59D for your Rx), or 0.86D. So, a pair of specs with 0.14D in each eye would pass muster. The ANSI standard is actually a bit more demanding in this instance (it would be 0.59D).

    Not that your glasses shouldn't be made correctly - but I'll go out on a limb and suggest that there are lots of happy spectacle wearers out there whose glasses conform less to their prescriptions than do yours.

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    OptiBoard Professional Mike Fretto's Avatar
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    If I'm reading the attached lab ticket it appears theres only 1mm vert. dec. which would result in 1/2 diopt prism not the prescribed 1.0. Also if I'm not mistaken all aspherics have whats refered to as a spherical button in the center that would allow some slight manipulation of the oc placement. The above RX as mentioned would only need 2mm vert dec to achieve the 1.0 diopter of prism needed. I'm not sure of the size of this spherical area but I wouldnt think 2mm would hurt. Anyway thats my 2 cents worth hopefully it helps.
    Mike

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    Bad address email on file John R's Avatar
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    Re: Can you feel it?

    shanbaum said:
    I don't recognize the lens "Nikon IV" - could it be the same as what we call the "NL4" over here?
    Well if they are using roman numerals then it sure could be as IV = 4.

    Yahya says in the first post he ordered Nikon NL4 AS (n=1.67)..Must be the labs way of describing them.

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    Objection! OptiBoard Gold Supporter shanbaum's Avatar
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    Mike Fretto said:
    If I'm reading the attached lab ticket it appears theres only 1mm vert. dec. which would result in 1/2 diopt prism not the prescribed 1.0.
    That's 1.0mm up to achieve the specified PRP height (15).

    As I said, the prism would have to have been spotted-in if it's going to be there.

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    Objection! OptiBoard Gold Supporter shanbaum's Avatar
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    Thumbs up Re: Re: Can you feel it?

    John R said:
    Well if they are using roman numerals then it sure could be as IV = 4.

    Yahya says in the first post he ordered Nikon NL4 AS (n=1.67)..Must be the labs way of describing them.
    OK, I see that now.

    The database on my PC is pretty old - maybe the 0.50 base has been added since I last updated it.

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    OptiBoard Professional Mike Fretto's Avatar
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    After a closer look at your workticket what they've done is raise the oc 1mm in each eye so there is no vert prism correction. The vertical measurement of the frame appears to be 28mm which would put the oc normally at 14 notice the oc height specified is 15mm as well as the vert decentration in finish layout is +1 ou.
    Mike

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    OptiBoard Professional yzf-r1's Avatar
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    re: oc height=15

    The vertical centration was set at 1 above datum (normal viewing position is 4 above datum, but with angle of side taken into account, this was set at 1 above datum.) So in a frame with a b measurement of 28, the centres were set at 15 above the bottom edge

    Yahya
    Last edited by yzf-r1; 09-27-2002 at 05:08 PM.

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    OptiBoard Professional Mike Fretto's Avatar
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    Both lenses were decentered 1mm up so it appears there was no attempt to grind the prism into the lenses through the surfacing process, nor was there any vertical prism achieved through decentration. I'm assuming your having problems with this pair of glasses????
    Mike

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    OptiBoard Professional yzf-r1's Avatar
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    Mike,

    Having re-read my last post, i realised that it was probably more confusing than i had intended.

    The raising of OCs to 1 above datum is there before taking into account any prism. i.e. if no prism was ordered, the centres would be at 1 above datum

    Indeed, my spectacles do have prism incorporated in them, but as i mentioned earlier, my problem arises from the fact that i thought the prism was incorrectly achieved

    Many thanks

    Yahya 'hopefully clearer' Vali

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    OptiBoard Professional yzf-r1's Avatar
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    Sorry, fellow O'Boarders for coming back to this post, but ever since i got my new spectacles i have become fascinated by the topic of prisms and aspheric lenses.

    If any of you remember, i posed a question as to whether there was a way of telling whether the prism on a lens has been created by decentration or by surfacing, and jeff gave me a really neat way of working that out.

    I have continuously pondered if there was another way of finding this out, and here it is. It seems to make sense in theory, and i have tried it on a couple of lenses, and it works for me, so i have decided to post it.

    Mark the optical centre of the lens using a focimeter. Now mark 2 points equidistant from the optical centre (such that the OC is exactly in the middle of these 2 points). Measure the thickness at each of these 2 points. If the thickness at these 2 points is the same, then the lenses have been decentred, and if there is a difference, then the prism has been worked on the lens.

    Any comments on my method?

    yahya 'i need to go out me' vali
    curiosity killed the cat...well, in that case i should be dead soon

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    Objection! OptiBoard Gold Supporter shanbaum's Avatar
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    Thumbs up Good job

    That's a vastly better way, but be sure that your measuring points are widely separated. Aspheric designs often have a large spherical zone - 20mm diameter or more - around the aspheric pole.

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    Master OptiBoarder Jeff Trail's Avatar
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    Robert is correct..BUT (I just love throwing that out there) the problem with your idea of making a "grid" is that it would work great if you were to have access to all the uncut blanks (before edging and mounting) but in smaller frames and PD's all over the place it may hamper your ability to get far enough away from the apex of the aspherical curve :-)
    One other thing, since we are "revisiting" this thread, you guys may have overlooked the obvious and took the surface ticket as exactly what it says to do and so both OC's were 1 above the datum and so no prism was in the lens..than came back with the answer "yes prism is in the lens I have"... that tells me they used the ticket for surfacing BUT tossed it out when they did the edging..they just decentered it according to the frame (the finisher) .. shoot I don't go by the ticket either when mounting lens...since the person putting the information in for surfacing might not get the same exact measurements as I do (plus I have a patternless edger that figures out the placement for me :) Another problem with some of the surface programs tied into a network with say the frame fax is the measurements the frame companies supply and what the frame ACTUAL measures are not that exact now adays.. I've seen the eye size vary as much as 2 mm from OD to OS and circumference ..YIKES that is a nightmare nowadays from OD to OS..
    When looking at a ticket you really need to know how the lab work is done by that certain lab to be able to get the correct answer... take prism thinning PAL's I do not let my surface program figure that but do it as I run it through the system, at times (I have tried three surface programs) they tend to leave to much E.T. in high plus RX's.
    Good trick though.. setting up your grid (oh and you really need to use digital calipers though) just would be better to use when the lens are in the uncut form not after edged and mounted.. specially in the "smaller the better" frame phase we seem to be in :-)

    Jeff "see what can be gained through curiosity" Trail :-)

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    OptiBoard Professional yzf-r1's Avatar
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    Re: Good job

    shanbaum said:
    That's a vastly better way
    Robert, i am so glad you said that. Makes all those hours of thinking about an alternative method well work it:D

    yahya
    curiosity killed the cat...well, in that case i should be dead soon

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